One of my lines have had this issue for a few years and now that I'm abroad for a few weeks, I want to get to the bottom of this. I brought 2 SIM cards and 2 phones with me, anticipating one of them will not have data:

Simple Choice Business account. Except for 1 line, all lines in the account can get international data,

1 Line cannot get International Data roaming at all. When abroad, "Welcome to Country! Your plan gives you unlmited data at up to 2G speeds..." SMS is received, but data doesn't work but voice and text work fine.

No international block on the line,

Not a phone configuration issue; not an APN/roaming/network settings issue:

Swapping another line's SIM into the very same phone provides that phone international data; everything works with the other SIM

Putting the problem line's SIM back into the phone brings back the issue

Account has had this issue with a previous phone; swapping another line's SIM also fixed the issue

Putting the problem line's SIM on a second phone doesn't resolve the issue (that phone works with the other SIM)

NOT A SIM issue either; I have previously swapped the SIM on this line following the advice of Tmo technician and it didn't resolve the issue.

Bottom line, it's a line issue; it seems like this line has something that prevents the overseas provider to accept roaming.

A few years ago, opened a technical ticket on this but it took too long to resolve and had gone back to the US

Being abroad with not much downtime, I can't be on the phone with CS agent. What's the best and quickest way to get this issue looked at by tier-3 or higher?

It sounds like you've done your homework, and I'd have to agree that it sounds like something on the backend system needs updated. With your situation in mind (being international at the moment), I'd recommend working with T-Force over Facebook or Twitter. You'll be able to have an ongoing conversation at your convenience.